1. Technical Field
The present invention relates to a radiation imaging apparatus that includes a pixel-type measurement system and images an incident radiation distribution, and to a nuclear medicine diagnosis apparatus that uses the radiation imaging apparatus.
2. Background Art
A gamma camera, single photon emission computed tomography (SPECT) apparatus that uses a gamma camera, or other nuclear medicine diagnosis apparatus is used as an apparatus that uses a radiation measurement device for medical purposes. Radiation detectors (hereinafter may be referred to as detectors) for use in such a nuclear medicine diagnosis apparatus are mostly a combination of a scintillator and a photomultiplier tube. For such a nuclear medicine diagnosis apparatus, a single, large crystal plate is generally used. A NaI (T1) scintillator is widely used for the gamma camera and SPECT apparatus.
FIG. 13 schematically illustrates the configuration of a scintillator-based gamma camera. A single plate of scintillator 201, which comprises a relatively large single crystal, is a detector that makes use of a phenomenon in which fluorescence is generated when radiation energy is absorbed subsequently to radiation incidence on a particular substance. The generated feeble light is amplified by a plurality of photomultiplier tubes 203 to achieve radiation detection. For radiation position measurement purposes, the output signals generated from the plurality of photomultiplier tubes 203 are subjected to gravity center computation to determine a radiation reaction position.
To project a gamma ray generation position onto an image pickup surface of the detector, a collimator 206 for controlling the angle of radiation incidence is positioned in front of the scintillator 201. At present, the collimator 206 is generally made of lead that has an infinite number of hexagonal through-holes. The through-hole diameter approximately ranges from 1 mm to 3 mm. The through-hole length approximately ranges from 40 mm to 60 mm. The septa among the through-holes approximately range from 0.2 mm to 3 mm. Hexagonal through-holes are used because they provide the highest aperture ratio, are easy to fabricate, and exhibit high strength. In FIG. 13, the reference numeral 202 denotes a light guide; 204, a measurement circuit; and 205, a measurement circuit retention board.
In recent years, individual pixel type detectors, which acquire position signals in the unit of a small detector, that is, on an individual pixel basis, have been developed, including a gamma camera in which a CsI (T1)-based pixel type scintillator and photodiode are used (Nuclear Medicine Examination Technology, Japanese Society of Radiological Technology, Ohmsha, pp. 79-80) and a semiconductor detector for directly converting radiation into electrical signals (Nuclear Medicine Examination Technology, Japanese Society of Radiological Technology, Ohmsha, pp. 76-77). Detectors that determine the radiation reaction position by means of aforementioned gravity center computation measure one gamma ray by using a plurality of photomultiplier tubes to capture scintillator-generated light as a spread of light. Therefore, it can be said that the detectors make spatially continuous measurements, that is, analog measurements. On the other hand, it can be said that pixel-type detectors, which make measurements on an individual pixel basis, measure one gamma ray by making spatially discrete measurements, that is, spatially digital measurements.
One measurement unit, that is, the radiation incidence cross section of a pixel, of the above apparatuses is generally rectangular. The collimator having hexagonal through-holes is not suitable for the above apparatuses. The reason is that moire patterns arise although they do not arise with the use of a conventional scintillator, which comprises a single crystal. The generation of moire patterns is a problem in which a plurality of periodical sensitivity variations occur on an image when the periodical shade changes of septa interfere with each pixel due to the difference between the detector pitch and through-hole pitch and anisotropy.
One solution to avoid moire patterns is to use a collimator whose hole diameter is smaller than half the pixel size. When the collimator through-hole is small, the following advantages are provided. When the collimator is shifted horizontally in relation to the detectors, a septum positioned over one detector is partly positioned outside the detector. However, another septum, which has virtually the same area and was positioned outside the detector, is now positioned over the detector. As a result, the septum area over the detector does not significantly change even when the collimator is shifted. Consequently, the detector sensitivity does not significantly change. In other words, the resulting image remains almost unchanged because pixels are almost uniform in sensitivity even when the collimator is moved forward, rearward, leftward, or rightward, rotated, or otherwise shifted. The smaller the collimator through-holes in relation to the detectors, the greater the produced effect.
However, when the pixel size is 1 mm or larger, the above solution does not work due to the manufacturing limitation imposed on the collimator hole diameter. As a result, moire patterns cannot be avoided.
Another solution is to use a matched collimator, which has rectangular holes that match the pixel size. Since the sensitivity loss due to the septa 28 is minimized for the pixel-type detectors, it is said that the use of a matched collimator is ideal. However, when the current lead-based collimator is used, it is difficult to maintain the manufacturing accuracy in order to provide the advantages of the matched collimator. The reason is that lead is relatively soft and likely to deform. Further, if, for instance, the collimator mounting position is slightly shifted from normal, a great sensitivity variation may arise. The collimator can be made of relatively hard tungsten in order to maintain the required manufacturing accuracy. Such a solution may work with collimators for use in a small-size gamma camera, but does not provide a practical solution for collimators for use in a normal gamma camera, SPECT, or the like in terms of cost.
Further, the gamma camera rotates or moves in a complicated manner during an image pickup operation. During such a movement, the collimator may deviate from a specified position.
Even while the gamma camera is at a standstill for a long period of time, the collimator may gradually deviate from a specified position due to its weight.
When displaced, the collimator incurs moire patterns no matter whether a matched collimator is used.